Flexible pipes of the present type for offshore transportation of fluids are well known in the art and are for example described in “Recommended Practice for Flexible Pipe”, API 17 B, second Edition, 1998, which provides a standard for such pipes. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armor layers on the outer side of the inner sealing sheath (outer armor layer(s)). Often the flexible pipe also comprises an armor layer on the inner side of the inner sealing sheath, such an armor layer is normally called a carcass and is mainly provided in order to reduce the risk of collapse of the pipe when used under high hydrostatic pressure. An outer sealing sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armor layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114, U.S. Pat. No. 6,085,799 and in Recommended Practice for Flexible Pipe, API, 17B, Fourth edition, July 2008, and in Specification for Unbonded Flexible Pipe, API, 17J, third edition, July 2008.
The term “unbonded” means in this text that at least two of the layers including the armor layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armor layers located outside the inner sealing sheath. These armor layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
The outer armor layer(s) often comprises one or more helically wound elongate elements, such as a wire, which for example can be profiled and optionally interlocked.
For example flexible pipes with a pressure armor layer of profiled and interlocked elongate elements are well known. U.S. Pat. No. 6,065,501 for example discloses a reinforced flexible tube with an armor layer of helically wound elongate reinforcing element with a T-shaped cross-sectional profile. The helically wound elongate reinforcing element with a T-shaped cross-sectional profile may be interlocked with another elongate element having a cross-section with a T-shape mounted in the reverse direction or with a corresponding U-shaped cross-sectional profile.
U.S. Pat. No. 6,889,717 discloses a flexible pipe with at pressure vault (pressure armor layer) with an external face and an internal face placed over the internal sheath, the vault consisting of the winding, in a helix with a short pitch and with a gap between turns, of a metal profile wire interlocked from below by a fastener wire, where the fastener wire has substantially the same height as the profile wire and is interlocked, with no nominal radial contact, below the neutral fiber of the wire (that is to say the fiber where there is no elongation during spiraling, at the centre of mass of the cross section of the wire). The profile wire may have I-shaped cross section or “psi” ψ shaped cross section. The fastener may have a T-shaped cross section.
A flexible pipe of the above type will often be subjected to high axial forces due to high pressure on the outer side of the pipe and/or due to high pressure in the bore of the pipe. Even in situations where an armor layer is made from interlocked profiles, these profiles may have a tendency to slide out over each other with subsequent damage to the pipe.
This problem has been solved with the construction described in U.S. Pat. No. 6,739,355. In this construction the armor layer comprises primary, spirally-wound profiles provided with at least two recesses in which a profile of another kind referred to as a locking profile, is placed, whereby a locking of the primary, spirally-wound profiles against mutual, uncontrolled, axial displacement is achieved. This structure is generally called the K-profile structure. In a variation thereof the locking profile has been integrated into the K profile, which along one of its edges is provided with a recess and along its other edge is provided with the locking profile in the form of a tongue corresponding to the recess. In this way, it becomes possible to avoid the use of locking elements, which can have certain advantages from the point of view of production technique.